The subject-matter of the present invention is temperature fuses which can be used, for example, in gas generators for motor vehicle safety systems.
Gas-generating mixtures used in gas generators for motor vehicle safety systems are, as a rule, thermally very stable. In order to ignite the gas-generating mixture in a controlled manner at high ambient temperature, for example in the case of a vehicle fire, so-called temperature fuses are used. Such a fuse is necessary in order to prevent the gas-generating mixture self-igniting in an uncontrolled manner at unusually high temperatures. Namely, at high temperatures, the gas-generating mixture would not burn normally, but because of the increased temperature would react in a correspondingly accelerated and violent manner, in unfavourable cases explosively. The generator housing is not designed for this accelerated, violent reaction and would thereby be destroyed. A considerable risk to the motor vehicle passengers would be the result. The temperature fuse ensures that the reaction of the gas-generating mixture is thermally triggered well below this critical temperature. As a result of its early reaction and controlled ignition of the gas-generating mixture in such a case, it prevents the destruction of the generator housing and the dangers linked therewith.
In the prior art, nitrocellulose, or propellent charge powder derived therefrom, are generally used as a temperature fuse. A crucial disadvantage of nitrocellulose, however, is that it already begins to decompose slowly at temperatures which are still not sufficient for ignition. In the extreme case, the nitrocellulose decomposes completely. It can then no longer fulfil its task as a temperature fuse. Attempts have admittedly been made to improve the thermal stability of nitrocellulose. These attempts, however, are subject to narrow restrictions.
An object of the present invention has therefore been to provide a temperature fuse which does not have the disadvantages of the nitrocellulose-based temperature fuse.
The underlying object of the invention was achieved by a temperature fuse having the characterizing features of the main claim. Advantageous developments are characterized in the subclaims.
Surprisingly, it has been established that the temperature fuses in accordance with the invention are able to ignite thermally the gas-generating mixtures normally used in gas generators in a controlled manner well below the critical temperature.
Substances or mixtures of substances which have lower detonation points or decomposition points than the actual gas-generating mixture can be used as temperature fuses in accordance with the invention. The absolute level of the detonation points or decomposition points of the temperature fuses in accordance with the invention thereby depends on the respective construction and housing stability of the gas generator which is used. The more stable the generator housing, for example, the higher in general these values can be for the temperature fuse in accordance with the invention.
Substances or mixtures of substances, the exothermal thermal decomposition of which takes place in a narrowly restricted temperature range, can be used for the temperature fuses in accordance with the invention. The evolution of heat which occurs must, in this case, be sufficient to compensate for energy losses in the gas-generating mixture, in order to achieve, or exceed, the activation energy needed to ignite the gas-generating mixture.
Compounds selected from the compound classes of oxalates, peroxidisulphates (persulphates), permanganates, nitrides, perborates, bismuthates, formates, nitrates, sulphamates, bromates or peroxides can be used as substances or mixtures of substances for the temperature fuses in accordance with the invention. As oxalates, there can preferably be used iron(II)oxalate dihydrate which has a distinct decomposition point at 190xc2x0 C., ammonium-iron-(III)-oxalate, the double salt of ammonium oxalate and iron oxalate with decomposition temperatures of 160-170xc2x0 C.; as peroxidisulphates (persulphates) preferably ammonium persulphate, sodium persulphate or potassium persulphate, the thermal decomposition of which is suitable for starting the reaction; as permanganates preferably sodium permanganate or potassium permanganate; as formates preferably ammonium formate or calcium formate; as a nitrate preferably ammonium nitrate; as a sulphamate preferably ammonium sulphamate; as a nitride preferably iron nitride, as a bismuthate preferably sodium bismuthate; as a bromate preferably potassium bromate, and as a peroxide preferably zinc peroxide. Iron oxide and/or ferrocene, can also be used. Apart from this, oxidizable components, for example explosives having low detonation or decomposition points, preferably calcium bistetrazolamine, 3-nitro-1,2,4-triazol-5-one (NTO), 5-aminotetrazole nitrate, nitroguanidine (NIGU), guanidine nitrate and bistetrazolamine, can be used. The substances can be used individually or in a mixture. A specific thermal decomposition point of the temperature fuse in accordance with the invention can be adjusted by adjusting the mixture.
Of these substances, those which have a lower detonation point or decomposition point than the gas-generating mixture which is used and thereby decompose exothermally can be used alone, without addition of a fuel, for example, as a temperature fuse in accordance with the invention. The substances which have a lower detonation point or decomposition point than the gas-generating mixture which is used but thereby decompose endothermally require at least one fuel and possibly a reducing agent in order to be able to be used as a temperature fuse in accordance with the invention. For example, the known explosives preferably calcium bistetrazolamine, 3-nitro-1,2,4-triazol-5-one (NTO), 5-aminotetrazole nitrate, nitroguanidine (NIGU), guanidine nitrate and bistetrazole amine can be used as fuels, and metal powder, preferably titanium powder, can be used as a reducing agent, for example.
When the explosives are used as a temperature fuse in accordance with the invention, having lower detonation or decomposition points than the gas-generating mixture which is used, then, in addition to the substances already mentioned above, guanidine nitrate, or even oxidizing agents such as potassium nitrate, sodium nitrate, strontium nitrate, potassium perchlorate or mixtures of these oxidizing agents, can be added in order to influence the detonation points and thus the range of effectiveness of the temperature fuse in accordance with the invention.
The temperature fuses in accordance with the invention can be used in a variety of ways. One use provides using them homogeneously in the gas-generating mixture. In particular, the temperature fuses in accordance with the invention that do not impair or impair only to an insignificant extent the actual characteristic of the gas-generating mixture are suitable herefor. The homogeneous distribution can take place according to mixing methods which are known per se, for example by sieving or tumbling the dry mixture or by kneading, extruding or extrusion moulding a moistened or solvent-containing mixture. The addition of a binding agent is likewise possible. In the case of this use, the temperature fuses in accordance with the invention can constitute 0.1 to 20% by weight, preferably 0.1 to 5 by weight, of the gas-generating mixture.
A further use provides for the temperature fuses in accordance with the invention in the generator housing to be separated from the actual gas-generating mixture. This use is always to be recommended if the temperature fuses in accordance with the invention that are used impair the actual characteristic of the gas-generating mixture. In the case of this use, these temperature fuses in accordance with the invention are preferably provided at thermally exposed points on the generator housing. In this way, a reliable triggering of the temperature fuse in the case of heating from the outside is ensured, as a result of which the controlled ignition of the gas-generating mixture is ensured. In the case of this use, the admixtures in accordance with the invention can be used in the form of tablets, for example. The production of such tablets takes place according to methods which are known per se.
A further use provides including the temperature fuses in accordance with the invention in the normal ignition device of the gas-generating mixture. In this case, two variants can be used: the temperature fuses in accordance with the invention are distributed homogeneously in the ignition mixture or are separated therefrom, for example in the form of a tablet.
In all applications, the purity of the substances which are used determines the instant of thermal triggering and the grain size determines the energy which is released locally. For improved processing of the temperature fuses in accordance with the invention, processing aids which are known per se, for example talc, graphite or boron nitride, can be used.
In addition to their use in safety systems, the temperature fuses in accordance with the invention can also be used, for example, in pressure or safety elements for triggering movements of mechanical elements.
The temperature fuses in accordance with the invention are compatible with the gas-generating mixture and its components and show, in accordance with the requirements, a temperature and storage stability that is sufficient for the instance of application and considerably improved in comparison with nitrocellulose. The problem of slow decomposition at comparatively high storage temperatures, which is to be noticed in the case of nitrocellulose, is not displayed by the temperature fuses in accordance with the invention. A thermal change at the required storage and operating temperatures could not be established.
The requirement for the substances used to be toxicologically safe is likewise fulfilled, as is the requirement for the gases and reaction products of the gas-generating mixture, which can be used when blowing up an air bag, for example, to be toxicologically safe; there is no need to fear the motor vehicle passengers being put at risk or harmed.
The disposal of the gas-generating mixture with the temperature fuses in accordance with the invention is also safe; it is ensured with simple means and without expensive installations.